The crystal materials of semiconductors, dielectrics, magnets, etc., for example, silicon semiconductors, compound semiconductors, or oxide single crystals such as lithium niobate, are developed and used as highly functional materials in the field of electronics or optoelectronics, so that they are becoming increasingly important.
These crystal materials are in general produced by the Czochralski method (hereinafter, referred to as the CZ method) as a single crystal in a form of a rod the single crystal being from a crucible in which raw materials for growing a single crystal are filled and melted.
It is generally known that constitute components (for example, oxygen) of a crucible, for example, a quartz crucible, as well as some impurities included in the raw materials, are incorporated into the pulled single crystal in the case where crystal growing is carried out by the CZ method.
The concentrations of the impurities incorporated into the pulled single crystal are dependent on the speed of crystal rotation, the speed of crucible rotation, the distribution of the temperature of the melt, etc., because the speed of crystal rotation has an influence on the melt convection flow, the speed of crucible rotation has an effect on the melt convection flow and the oxygen concentration in the melt itself, and the distribution of the temperature of the melt has an influence on the melt convection flow. Therefore, it is possible to control the concentrations of the impurities to some extent by controlling these factors.
It is, however, difficult to control the melt convection flow, so that the melt convection flow can not be sufficiently controlled only by the above factors.
Then, a method where a single crystal is pulled by the CZ method, with applying a magnetic field to the melt, the method being called the magnetic-field-applied Czochralski method (hereinafter, referred to as the MCZ method) was developed.
In a single crystal produced by the MCZ method, the concentrations of the impurities in the crystal, for example, the oxygen concentration can be more extensively and more precisely controlled than that in a single crystal produced by the CZ method, and swirl defects and striations, etc. are also decreased, when the melt has electric conductivity as silicon melt. This is due to the suppression of the thermal convection in the melt by increasing the effective viscosity of the melt by applying a magnetic field.
On the other hand, single crystal materials must meet more and more requirements due to high-precision and high-integration, etc. of devices such as semiconductors. Regarding the concentrations of impurities in a single crystal, it is known, for example, that the oxygen concentration and distribution in a semiconductor silicon single crystal have great influences on the characteristics of the semiconductor devices from the silicon single crystal. That is, if the oxygen concentration is too high, crystal defects or oxide precipitates generate and have various harmful effects on the characteristics of the semiconductor devices. However, if such crystal defects or oxide precipitates generate in the region except for the active region of the semiconductor devices, those behave as gettering sites of heavy metal impurities, so that the characteristics of the semiconductor devices can be improved ("intrinsic gettering"). Therefore, if the oxygen concentration is too low, the characteristics of the device can not be improved.
Accordingly, the crystal materials are required to contain the sorts and concentrations of impurities to be desired depending on the type of a device to be produced, and the allowable ranges of the concentrations thereof is becoming remarkably limited. At present, it is impossible to highly control the concentrations of the impurities in a single crystal and to meet such strict requirements by simply using the MCZ method.
The present invention was made in view of the foregoing, and therefore an object of the present invention is to highly precisely control the concentrations of the impurities in a single crystal to be grown.